Hi Buz.
Welcome back. (Even though I have changed, I respect your position entirely, and understand why you stick to your guns, and I have no problem with your belief or ideological position).
.
If there were something stopping 50 + 50 making 100 then maybe there would be a debate. The only reason to suggest the two terms, would be because it would favour the creationist position.
I have thought at times, that some kind of barrier was needed, for the protection of a species. Brad might have thought of this, in that I thought that some kind of basic protection of the working morphology would be intact. But when you think about it, this would hinder the ability to adapt over time in order to survive, anyway.
Small changes over a very long time. I don't have much sense of size myself, but really, try and imagine the time-spans we're talking about here. They're vast. We're the last second at midnight.
If evolutionits said, yeah - okay, speciation isn't enough. Then that would portray a false picture of the evidence of clear transitionary forms such as homo floresiensis, Ergaster, Erectus, etc.., and the clear evidence of one species from the genus leading to the next, etc.. What stops this? The evidence shows nothing, and even AIG admitt rapid evolution, they just don't admitt that we would be in the ape-kind.
I used to think one falsification would be enough. But there are none. The true "barrier" is infact workable through a none-barrier. A very clever God would say that normalized selection solves the problem. A none-barrier as a barrier. If your form is great, keep it, if not evolve when a mutation or trait allows you to.
I suspect your problem is imagining basic organs and vital components coming about over time, even complete systemry. Mine too, but I have to learn more if I want to understand how. Unfortunately I am biologically incapable, as I am no good at the subject.